package org.apache.lucene.index;

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import java.io.IOException;
import java.util.Arrays;

import org.apache.lucene.store.IndexOutput;

/**
 * Implements the skip list writer for the default posting list format
 * that stores positions and payloads.
 *
 */
class DefaultSkipListWriter extends MultiLevelSkipListWriter {
    private int[] lastSkipDoc;
    private int[] lastSkipPayloadLength;
    private long[] lastSkipFreqPointer;
    private long[] lastSkipProxPointer;

    private IndexOutput freqOutput;
    private IndexOutput proxOutput;

    private int curDoc;
    private boolean curStorePayloads;
    private int curPayloadLength;
    private long curFreqPointer;
    private long curProxPointer;

    DefaultSkipListWriter(int skipInterval, int numberOfSkipLevels, int docCount, IndexOutput freqOutput, IndexOutput proxOutput) {
        super(skipInterval, numberOfSkipLevels, docCount);
        this.freqOutput = freqOutput;
        this.proxOutput = proxOutput;

        lastSkipDoc = new int[numberOfSkipLevels];
        lastSkipPayloadLength = new int[numberOfSkipLevels];
        lastSkipFreqPointer = new long[numberOfSkipLevels];
        lastSkipProxPointer = new long[numberOfSkipLevels];
    }

    void setFreqOutput(IndexOutput freqOutput) {
        this.freqOutput = freqOutput;
    }

    void setProxOutput(IndexOutput proxOutput) {
        this.proxOutput = proxOutput;
    }

    /**
     * Sets the values for the current skip data. 
     */
    void setSkipData(int doc, boolean storePayloads, int payloadLength) {
        this.curDoc = doc;
        this.curStorePayloads = storePayloads;
        this.curPayloadLength = payloadLength;
        this.curFreqPointer = freqOutput.getFilePointer();
        if (proxOutput != null)
            this.curProxPointer = proxOutput.getFilePointer();
    }

    @Override
    protected void resetSkip() {
        super.resetSkip();
        Arrays.fill(lastSkipDoc, 0);
        Arrays.fill(lastSkipPayloadLength, -1); // we don't have to write the first length in the skip list
        Arrays.fill(lastSkipFreqPointer, freqOutput.getFilePointer());
        if (proxOutput != null)
            Arrays.fill(lastSkipProxPointer, proxOutput.getFilePointer());
    }

    @Override
    protected void writeSkipData(int level, IndexOutput skipBuffer) throws IOException {
        // To efficiently store payloads in the posting lists we do not store the length of
        // every payload. Instead we omit the length for a payload if the previous payload had
        // the same length.
        // However, in order to support skipping the payload length at every skip point must be known.
        // So we use the same length encoding that we use for the posting lists for the skip data as well:
        // Case 1: current field does not store payloads
        //           SkipDatum                 --> DocSkip, FreqSkip, ProxSkip
        //           DocSkip,FreqSkip,ProxSkip --> VInt
        //           DocSkip records the document number before every SkipInterval th  document in TermFreqs. 
        //           Document numbers are represented as differences from the previous value in the sequence.
        // Case 2: current field stores payloads
        //           SkipDatum                 --> DocSkip, PayloadLength?, FreqSkip,ProxSkip
        //           DocSkip,FreqSkip,ProxSkip --> VInt
        //           PayloadLength             --> VInt    
        //         In this case DocSkip/2 is the difference between
        //         the current and the previous value. If DocSkip
        //         is odd, then a PayloadLength encoded as VInt follows,
        //         if DocSkip is even, then it is assumed that the
        //         current payload length equals the length at the previous
        //         skip point
        if (curStorePayloads) {
            int delta = curDoc - lastSkipDoc[level];
            if (curPayloadLength == lastSkipPayloadLength[level]) {
                // the current payload length equals the length at the previous skip point,
                // so we don't store the length again
                skipBuffer.writeVInt(delta * 2);
            } else {
                // the payload length is different from the previous one. We shift the DocSkip, 
                // set the lowest bit and store the current payload length as VInt.
                skipBuffer.writeVInt(delta * 2 + 1);
                skipBuffer.writeVInt(curPayloadLength);
                lastSkipPayloadLength[level] = curPayloadLength;
            }
        } else {
            // current field does not store payloads
            skipBuffer.writeVInt(curDoc - lastSkipDoc[level]);
        }
        skipBuffer.writeVInt((int) (curFreqPointer - lastSkipFreqPointer[level]));
        skipBuffer.writeVInt((int) (curProxPointer - lastSkipProxPointer[level]));

        lastSkipDoc[level] = curDoc;
        //System.out.println("write doc at level " + level + ": " + curDoc);

        lastSkipFreqPointer[level] = curFreqPointer;
        lastSkipProxPointer[level] = curProxPointer;
    }

}
